H frame supported by arducopter ?

Tom and Hughes are mostly correct since you are not talking about a hexa-H or octa-H. While the latter sort of fly OK using X, I'm hoping to write documentation on how to do those.

Back to your issue. Your insight is partially correct. It depends on how you structure the H. What Tom and Hughes are saying is that for the copter to fly correctly, you need to structure the quad so that when lift is applied to prop 1, it also lifts all but the opposing prop proportionally. That is why an X is used on quads (the most efficient structural method to gain cross rotor stability). On an octa, the H becomes the most efficient structural design but the mid part horizontal dash on the H is actually an X. Without the X, you don't get cross rotor stability.

With a quad H, the problem comes in where the horizontal piece connects to the vertical piece (pretend your copter was flying straight up). That connection, if perfectly rigid would do what Tom and Hughes are saying. But as you pointed out, that isn't reality. It's not rigid even if bonded. The horizontal piece will twist putting high loads on that joint and a spring effect that will make PID tuning interesting.

My advice. On a quad, go with an X frame. It will weigh less. However, make one key modification. Most X designs break the X at the center so all the X parts have a length of the radius of the circle. Then you have to sandwich those arms between two platforms all connected with a bunch of weighty fasteners. Instead don't cut your cross beams in half. Keep them whole. Put one on top of a platform and one on bottom (offset so they stay whole). You only need one platform and half the number of fasteners. In fact, I just use zip ties. Lighter and stronger than conventional X assemblies. While you can put two props raised off of the lower arm so all are at an equal height, that isn't necessary from a flight characteristic point of view (the aspect created isn't large enough to matter).

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MR60

Forrest Frantz April 23, 2013 at 12:55pm

Tom and Hughes are mostly correct since you are not talking about a hexa-H or octa-H. While the latter sort of fly OK using X, I'm hoping to write documentation on how to do those.

Back to your issue. Your insight is partially correct. It depends on how you structure the H. What Tom and Hughes are saying is that for the copter to fly correctly, you need to structure the quad so that when lift is applied to prop 1, it also lifts all but the opposing prop proportionally. That is why an X is used on quads (the most efficient structural method to gain cross rotor stability). On an octa, the H becomes the most efficient structural design but the mid part horizontal dash on the H is actually an X. Without the X, you don't get cross rotor stability.

With a quad H, the problem comes in where the horizontal piece connects to the vertical piece (pretend your copter was flying straight up). That connection, if perfectly rigid would do what Tom and Hughes are saying. But as you pointed out, that isn't reality. It's not rigid even if bonded. The horizontal piece will twist putting high loads on that joint and a spring effect that will make PID tuning interesting.

My advice. On a quad, go with an X frame. It will weigh less. However, make one key modification. Most X designs break the X at the center so all the X parts have a length of the radius of the circle. Then you have to sandwich those arms between two platforms all connected with a bunch of weighty fasteners. Instead don't cut your cross beams in half. Keep them whole. Put one on top of a platform and one on bottom (offset so they stay whole). You only need one platform and half the number of fasteners. In fact, I just use zip ties. Lighter and stronger than conventional X assemblies. While you can put two props raised off of the lower arm so all are at an equal height, that isn't necessary from a flight characteristic point of view (the aspect created isn't large enough to matter).
Tom Rearick April 23, 2013 at 11:44am

An H is an X with a fat middle. So far as Arducopter is concerned, they are the same. You could mount 4 motors equidistant from each other on a hoola hoop. If there are two motors in the front, it is an X. If there is only one motor in the front, it is a +. It doesn't matter what shape the frame is - only the positioning of the motors.

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